1. Field of the Invention
The invention relates to a level sensor, in particular for the fuel tank of a motor vehicle, in which a resistor network is arranged on a fixed support, it being possible to pick off an output signal on the resistor network, which signal corresponds to the position of a float which follows the level of a liquids.
2. Description of the Related Art
Level sensors of this nature are known for use in motor-vehicle fuel tanks in connection with delivery units. In the installed state, such delivery units are supported on the base of the vehicle fuel tank. The level sensor is in this case arranged directly on the splash pot or is arranged on a sensor support which in turn is attached to the splash pot. A slide potentiometer of the level sensor is actuated by a float, vial a float lever, as a function of the level in the fuel tank. The changing level allows a corresponding electrical signal to be picked up at the slide potentiometer.
WO 94/27119 A1 has disclosed a level sensor with a variable resistance. A float is used to move a sensor magnet which is connected to a contact structure, the contact elements of which slide along a slide potentiometer. The resistor network of the slide potentiometer is arranged on a circuit board, it being possible to pick up on the resistor network an output signal which is dependent on the level of the liquid.
A variable resistor which comprises a resistor layer covering its entire surface and a magnetic contact structure which is arranged at a distance from this resistor layer is known from the citation WO 81/03077 for suppressing the drawback of wear to the mechanical components of such a slide potentiometer. Depending on the position of a magnet, parts of the contact structure are pulled onto the resistor layer and thus form an electrical connection. It is difficult to seal such a device with respect to external influences.
The invention is therefore based on the object of providing a level sensor which operates with low wear, is inexpensive to produce, has a high level of accuracy and is sealed with respect to external influences.
The advantage of this level sensor consists in the fact that it operates entirely without wear. Furthermore, it has a high level of contact reliability and a high resolution. The contact structure may be any structure which has any form of tongue-like spring elements, irrespective of whether these spring elements are attached individually or a plurality of spring elements are combined as a single-piece structure.
The improvement in the contact is brought about by the contact surfaces which have been applied to the substrate, with the result that the position sensor is able to acquire a vibration-free, robust structure of only small dimensions, which is particularly advantageous for use in motor vehicles.
Since the insulating substrate is used as the housing wall, it is possible, with only a few components, to produce a level sensor which is insensitive to aggressive environmental conditions and which allows the level sensor to be immersed in the fuel.
In a refinement, conductor tracks are arranged on the substrate, the end of each conductor track forming a contact surface.
The contact structure is arranged at a constant distance from the contact surfaces, which come into contact with the contact structure under the action of the magnetic device. The contact structure may in this case be a contact spring structure or may comprise separate contact springs. However, the contact spring structure may also be a single-piece flexural bar structure.
In one embodiment, the resistor network is designed as a resistor track in layer form and may be produced using thin-film or thick-film technology. The conductor tracks are partially covered by the resistor track, and the end of each conductor track in this case forms the contact surface.
Advantageously, at least the contact surfaces and the contact structure are enclosed in a sealed housing, and the magnet device is arranged so as to be movable outside the sealed housing. Such a level sensor does not have any open contacts with respect to the surrounding medium.
A reliable method of operation can be achieved if the magnet device is preloaded toward the outside of the housing, in such a manner that it can move in gentle contact therewith.